In this SBIR phase I grant application, Codex BioSolutions proposes to construct a cell-based high throughput assay that measures chemical toxicity using a novel dye, m-MPI. This dye was engineered to overcome many of the drawbacks of current dyes used to measure mitochondrial membrane potential. The parent dye JC-1 has been used for this purpose previously. In the mitochondria, where it accumulates as aggregates, it fluoresces red, whereas in the cytoplasm where it is a monomer, it fluoresces green. However, JC-1 is difficult to use because it has very low water solubility which makes using its differential fluorescent properties difficult. Our dye, m-MPI has increased solubility allowing easy quantitative assessment of MMP status simply by calculating the ratio of red/green fluorescence. m-MPI can also be used with cells that do not work with JC-1 (such as primary cells) due at least in part to its increased sensitivity. We believe our assay will more accurately assess hazards in a shorter time and with less cost.